1. Field of the Invention
The present invention relates, in general, to removal of core decay heat in a pool type liquid metal reactor which uses liquid sodium as a coolant when a normal heat removal system breaks down and, more particularly, to a direct pool cooling type passive safety grade decay heat removal method and system for a liquid metal reactor, which is capable of providing a large heat removal capacity suitable in design of a large thermal rated liquid metal reactor, and for minimizing heat loss during the normal plant operation while improving operational reliability.
2. Description of the Related Art
A general liquid metal reactor (LMR) is provided with a residual heat removal system (RHRS) for removing core decay heat arising due to urgent shutdown of the reactor when a normal heat removal system, which is formed through a reactor core, primary heat transport system (PHTS), an intermediate heat exchanger (IHX), intermediate heat transport system (IHTS) and a steam generator system (SGS), breaks down.
A conventional residual heat removal system for a pool type liquid metal reactor is designed to effectively remove core decay heat using thermal inertia of a hot pool disposed above a core outlet. The conventional residual heat removal systems are generally classified into the passive vessel cooling system (PVCS) and the direct reactor cooling system (DRCS) according to a residual heat removal capacity on the basis of thermal output of the core of a liquid metal reactor.
FIG. 9 shows the passive vessel cooling system (PVCS). When a normal heat removal system breaks down, sodium in a hot pool 150 is heated, and accordingly expanded. The expansion of the sodium raises its liquid level X1 above an overflow slot on a reactor baffle 130. As hot sodium heated in a core 110 flows over the overflow slot, it makes direct contact with a reactor vessel 100 so that convection and conduction heat transfer is performed between the hot sodium and the reactor vessel 100. In this way, core decay heat is removed. The passive vessel cooling system is a system applicable to small and medium thermal rated pool type liquid metal reactors with relatively low core heat output of 1,000 MWth or less.
Specifically, the heat absorbed into the reactor vessel 100 by means of the convection and the conduction is transmitted to a containment vessel 230 disposed outside the reactor vessel 100 by means of thermal radiation. The heat of the containment vessel 230 is absorbed by air flowing through an air channel radially divided by an air separator 220 disposed between the containment vessel 230 and a reactor support wall made of concrete and surrounding the containment vessel 230. Finally, the air heated in the air channel inside the air separator 220 is continuously discharged into the atmosphere, and external cold air is continuously introduced along the air channel outside the air separator. Through natural circulation of air as described above, the core decay heat is passively and continuously removed.
The passive vessel cooling system requires neither operator action nor any active component actuation when the normal heat removal system breaks down. Consequently, this system has an advantage in that it adopts a completely passive concept, by which operational reliability is guaranteed. However, the passive vessel cooling system is not applicable to a large thermal rated reactor since it can only be suitably used in a liquid metal reactor with relatively low core heat output of 1,000 MWth or less, as mentioned above, considering economical efficiency based on heat transfer surface area determined by the diameter of the reactor vessel and the related requirement for accommodating components in the pool.
FIG. 10 shows the direct reactor cooling system (DRCS). As shown in FIG. 10, the direct reactor cooling system comprises a sodium-sodium heat exchanger 20′ disposed in a hot pool 150 in such a manner that it is below the liquid level X2 of hot sodium in the hot pool 150, a sodium-air heat exchanger 40′ disposed on a reactor building, and a heat removing sodium loop 30′ connected between the sodium-sodium heat exchanger 20′ and the sodium-air heat exchanger 40′. The direct reactor cooling system is a system for discharging heat into a final heat sink, i.e., the atmosphere through natural circulation of sodium using density difference in the heat removing sodium loop 30′ formed by elevation difference between a heat inflow part and a heat sink part. The direct reactor cooling system has advantages in that it is not restricted by heat output of the core unlike the aforesaid passive vessel cooling system, and in that it provides a sufficient decay heat removal capacity required according to the goal of design.
In the direct reactor cooling system (DRCS), however, heat must be continuously supplied even in the normal plant operation in order to prevent solidification of liquid sodium in the heat removing sodium loop 30′ when the heat is transmitted from the hot pool 150 to the sodium-air heat exchanger 40′ via the heat removing sodium loop 30′. Such heat supplied during the normal plant operation is considered as a heat loss of a pool type liquid metal reactor system. Consequently, the direct reactor cooling system is designed to have the following components to minimize the heat loss during the normal steady-state conditions. In an air flow inlet 43′, through which air is introduced into the sodium-air heat exchanger 40′, and an air flow outlet 47′, through which air is discharged from the sodium-air heat exchanger 40′, are disposed dampers 170, respectively. In addition, isolation valves 180 are mounted in the heat removing sodium loop 30′. The flow rate of sodium and air is controlled by proper manipulations for the opening fraction of the dampers 170 and the isolation valves 180 so that the minimum amount of heat necessary to prevent solidification of the liquid sodium is supplied to the heat removing sodium loop 30′ during the normal plant operation. Consequently, the heat loss is minimized during the normal plant operation of the hot pool. When the normal heat removal system breaks down, the dampers 170 and the isolation valves 180 are opened to the maximum extent so that the core decay heat is effectively removed.
As described above, the isolation valves 180 are disposed in the heat removing sodium loop 30′, and the dampers 170 are disposed in the air flow inlet 43′ and the air flow outlet 47′, so that the opening fraction of the isolation valves 180 and the dampers is controlled to supply proper amount of heat necessary both for minimizing a heat loss in the normal plant operation and for preventing solidification of sodium in the heat removing sodium loop 30′. To increase operational reliability of the decay heat removal system during system transient conditions, the isolation valves 180 and the dampers 170 are designed with a specific safety grade so that the direct reactor cooling system has a passive concept. In the direct reactor cooling system, however, mechanical driving requirements of the isolation valves 180 and the dampers 170 must be satisfied, which means that decay heat removal function on the basis of the completely passive concept is impossible. Furthermore, the direct reactor cooling system (DRCS) is inferior to the passive vessel cooling system (PVCS) in terms of operational safety related to operational reliability of the decay heat removal system.